SPACE OBSERVATION WITH GESTRA
The high dependence of modern society on space-based infrastructure calls for seamless operational and radar-supported space surveillance. The first step in achieving this sensor suite was taken by the aerospace management of the German Aerospace Center who commissioned Fraunhofer FHR with the development, construction and testing of a high-performance, long-range phased array radar. This sensor, which was designed as a scientific demonstrator, forms the basis for future scientific investigations within the framework of the cooperation between the participating institutions.
Demands placed on the radar sensor
The surveillance of the low Earth orbit to an orbital height of 3,000 km for the purpose of detecting all objects and debris with the lowest possible radar cross section can only be realized with an extremely high-performance, phased array-based radar system. Detected objects are tracked in a track-while-scan-mode with special track beams to generate basic information for the description of the respective orbit on the basis of the generated tracklets. The detection of all detectable objects in the observation area necessitates a finely optimized strategy for the selection of the wave and beam forms combined with the transmit and receive beams depending on the temporal conditions arising from the high speeds of the debris particles.
The radar sensor, which will be designed, developed and constructed in accordance with the client's product assurance requirements, should have the capacity to conduct experiments over an operational life of at least 12 years. As a certain degree of flexibility is required with regard to the installation site to facilitate future system expansion, the sensor will have a partially mobile construction.
Realization of GESTRA system at Fraunhofer FHR
Due to the high transmit power, the system will have a quasi-monostatic design, i.e. the transmission and receiver systems will each be integrated into separate containers with dimensions of 18 x 4 x 4 m3 which will be positioned approx. 100 m apart. To facilitate the selection of special surveillance areas outside of the scanning range of the electronic beams when operating in certain modes, the two planar phased array antennas will be mounted on 3-axis positioners with a large rotation angle range. For maintenance and transport purposes, the 24-ton antenna/positioning unit can be moved from the interior to the operating position above the container with a scissor lift table. A custom-made radome with lowest insertion loss protects the antennas in all weather conditions.
Both antenna apertures comprise 256 active cavity-backed stacked patch antennas in a circular arrangement, whereby the individual transmit beams are implemented with linear polarization and the individual receive elements with double polarization. The dedicated transmit and receive modules for the individual elements are mounted on planks on the rear side of the antenna plates. These sub-units comprise 3 and 4 modules respectively, and the corresponding control units and decentralized power supply units allow easy maintenance of the antenna front-end. The planks are fed with electrical energy and control signals via ultra-large backplane circuit boards.
Pulsed transmitter modules with a high pulse output and a pulse-to-pulse ratio that is adapted for the waveforms were developed for the transmission system.The available bandwidth exceeds 100 MHz in L-band in accordance with the ITU requirements for outer band radiation. The required liquid cooling of the modules is implemented with a water distribution network which is integrated in the individual plank plates as well as in the antenna plate with a diameter of three meters. High-performance condensator units on the transmission plank guarantee sufficient energy storage for the emission of the long transmission pulses. The high total cooling capacity in the container is implemented by means of a chiller with two powerful compressors within the primary cooling circuit and subsequently transferred to the outdoor environment via ventilation units. An additional oil-free compressor flushes all of the 32 decentralized 8kW-DC/DC converters (700 V to 51 V) in the antenna with cooling air. An energy chain, which is integrated in the positioner and is essential for wide-angle azimuth, elevation and polarization rotation of the antenna, supplies the positioner/antenna front-end with approx. 92 water hose, heavy-duty cable, compressed air, high frequency and control line connections.
The receiver system has an identical liquid cooling system to guarantee a low noise figure independent of the ambient temperatures. It is based on the Software Defined Radio principle, whereby the receive signal is scanned at each element of the carrier frequency level. The corresponding receive module has two identical analog microwave preamplification paths with adjustable amplification and filter middle frequency. The dual channel, 12-bit, A/D converter sends the digitalized receive signals of both polarizations to the central FPGA. The implemented programmable firmware allows digital down conversion, baseband filtering and the first beamforming evaluation level. This facilitates the flexible programming of the radar's operating frequency. The optical digital outputs of all 256 receive signals are consolidated on one beamformer board and produce the receive patterns of the receive antenna which are freely programmable with regard to form and direction. This digital multiple beamforming in combination with an artificial expansion of the transmit beam is essential to meet the temporal requirements of space surveillance in the specified volume. Several of the received radar pulses then undergo S/N enhancing signal processing to increase the detection capacity of the radar system beyond the basic parameters defined with transmit power and noise figure. The complex signal algorithm needed to achieve this is implemented in a high-performance parallel computing system, i.e. the radar processor, with 40 kW power dissipation.
The future operation of the GESTRA system from the space situational awareness center via remote control will necessitate the continuous monitoring of all system-relevant subsystems and processes as well as ongoing monitoring of the condition of all components. More than 2000 sensors monitor temperature, air humidity, air and water pressure and coolant flow in both containers to guarantee safe radar operation at all times or, if necessary, prompt deactivation. Including the operational infrastructure, each container weighs approximately 90 tons.
The intention to use the system on a long-term basis is underlined by the client's (DLR-RFM) detailed instructions with regard to the observance of vigorous standards on product safety, quality management, documentation and verification, which are based on the ECSS standards of the European space agencies.
Fraunhofer FHR successfully completed the Critical Design Review for the GESTRA system in 2016. As a result, all of the described subsystems can now be realized and the corresponding firmware versions and algorithms optimized. The integration of the subsystems in both containers, which commenced in March 2017, will be followed by the verification of all monitoring and control tasks. The handover of the radar system to the Space Situational Awareness Center in Uedem is planned for 2018.